In back-end semiconductor assembly processes involving electronic devices such as vertical light-emitting diode (“LED”) devices, thermosonic ball bonding is a primary process used for forming electrical interconnections. Vertical LEDs are mounted on substrates arranged in a vertical orientation in a magazine and fed into a bonding machine vertically for bonding. Bonding is carried out to electrically connect an LED mounted on an edge of a standing substrate, such as a leadframe, to the substrate itself. Vertical LEDs have several advantages over more conventional lateral LEDs. Current spreading is significantly enhanced in vertical LEDs which may lead to reduced series resistance as compared to lateral LEDs. As a result, the light output and power conversion efficiency of vertical LEDs are also greatly improved at high injection currents. Furthermore, vertical LEDs show minimal degradation of optical power when subjected to a stress test at a current of about 400 mA, whereas the same stress will result in the destruction of the lateral LED due to increased current crowding and self-heating.
To connect an LED to a leadframe in a vertical LED device, a thin wire such as gold wire of 20 to 75 microns in diameter is bonded onto a circuit pad on the LED before bonding onto a corresponding lead of the leadframe on which the LED is mounted. In order to transport successive vertical LED devices into a bonding area, a leadframe transportation device including a linear indexing system or an indexer transports a leadframe at high speed along a leadframe transporting track or rail. As the speed of wire bonding vertical LEDs increases, it is desirable to correspondingly increase the operational speeds of subsystems working in conjunction with a wire bonding machine.
Conventional leadframe transportation devices for transporting vertical LEDs may use a single track leadframe feeding system to index a leadframe so that bonding and indexing are performed in series. The bonding system is idle when an indexer pushes out a bonded leadframe before feeding an unbonded leadframe to a bonding site. The idling time of the bonding system may increase as much as 30% when the bonding speed is high.
Leadframe transportation devices incorporating a dual-track leadframe feeding system have been devised to reduce the idling time of the bonding system as compared to when a single track leadframe feeding system is used. Two dual-track leadframe feeding systems are illustrated in FIGS. 1 and 2. FIG. 1 shows a top view of a conventional synchronous dual-track leadframe feeding system 100 incorporating a synchronous indexer 102. The synchronous dual-track leadframe feeding system 100 comprises a first track or front leadframe rail 108 and a second track or rear leadframe rail 110 which is fixedly located parallel to the front leadframe rail 108 at a predetermined distance from the front leadframe rail 108 such that the pitch of the rails 109 is fixed. There is also a pre-heater block 104 and a heater block 106 with bonding sites located at opposite sides of the heater block 106, and a bonding system 107 that is locatable over the two bonding sites.
An indexing actuator activates the synchronous indexer 102 to feed an unbonded leadframe 112 each onto the front leadframe rail 108 and the rear leadframe rail 110 from a leadframe magazine located at a first end of the synchronous dual-track leadframe feeding system 100. The two unbonded leadframes 112 are indexed together along the dual tracks towards the opposite end of the tracks and are heated at the pre-heater block 104 before being indexed to the heater block 106 for further heating before bonding. The bonding system 107 is movable between the front leadframe rail 108 and the rear leadframe rail 110 to perform bonding at each leadframe 112. After the vertical LEDs on the two bonded leadframes 112 have been completely bonded, the two bonded leadframes are indexed together downstream and unloaded into a magazine. At the same time, another two unbonded leadframes 112 are fed to the first end of the dual tracks 108, 110. The bonding system 107 would be idle and unproductive during the loading and unloading of the leadframes 112 to and from the synchronous dual-track leadframe feeding system 100. Thus, the synchronous dual-track leadframe feeding system 100 still relies on a series bonding and indexing sequence as the single track feeding system described above. The throughput gain over a conventional single track feeding system is hence minimal.
Moreover, since the pitch distance between the dual tracks is fixed, the synchronous dual-track leadframe feeding system 100 is customized based on the pitch of a selected magazine. This limits the flexibility of handling magazines of varying pitches. At the same time, having only a single pre-heater block 104 and a single heater block 106 between the first and second tracks 108, 110 which are in contact with the dual tracks causes the transmission of undesirable vibrations resulting from the opening or closing of leadframe clamps on each leadframe from one track to the other. Hence, wire bonding at one leadframe 112 is preferably stopped when another leadframe 112 is being clamped or unclamped, otherwise inaccurate bonding will result.
FIG. 2 shows a top view of an improved conventional buffer table dual-track leadframe feeding system 100′ incorporating two buffer tables comprising a buffer loader 114 and a buffer unloader 116. The dual tracks comprise front and rear leadframe rails 118, 120 arranged parallel to each other. The front leadframe rail 118 is in contact with a front pre-heater block 117 and a front heater block 119. A rear pre-heater block 121 and a rear heater block 123 are located on the rear leadframe rail 120 and are separated from the front pre-heater block 117 and the front heater block 119, hence vibrations from the opening or closing of leadframe clamps on one leadframe will not affect the leadframe on the other. Wire bonding of a leadframe does not have to stop during clamping or unclamping of the other leadframe unlike in the prior art described above.
The front and rear leadframe rails 118, 120 each receives a leadframe 112 from an input magazine 122 via the buffer loader 114 using a buffer loader indexer 113 and either a front indexer 128 or a rear indexer 130. A bonding site is located adjacent to each track so that wire bonding may be carried out alternately at each bonding site. Each bonded leadframe 112 is unloaded to an output magazine 124 via the buffer unloader 116 using a buffer unloader indexer 125 and either the front indexer 128 or the rear indexer 130. The buffer loader 114 is movable along a first rail 115 for aligning itself with the input magazine 122 and either the front or rear leadframe rails 118, 120 for transferring one unbonded leadframe 112 each time from the input magazine 122 to either the front or rear leadframe rails 118, 120. Likewise, the buffer unloader 116 is movable along a second rail 127 for aligning itself with the output magazine 124 and either the front or rear leadframe rails 118, 120 for transferring one bonded leadframe 112 each time from either the front or rear leadframe rails 118, 120 to the output magazine 124. Hence, the operation speed of the buffer loader 114 and the buffer unloader 116 affects the throughput of the system significantly and causes a bottleneck in the bonding operation since bonding speed is generally very fast. The idling time of the bonding system 126 during feeding is comparatively long as compared to the bonding time. Furthermore, since loading and unloading of leadframes 112 to and from the front or rear leadframe rails 118, 120 require two buffer tables, when one buffer table breaks down, the whole buffer table dual-track leadframe feeding system 100′ cannot continue to operate and bonding must stop.
Additionally, the length of the buffer loader 114 and the buffer unloader 116 must be sufficient to accommodate the whole length of the leadframes. Increasing the length of the two buffer tables to accommodate longer leadframes will correspondingly increase the machine length. This results in a larger footprint which takes up valuable space. The buffer table dual-track leadframe feeding system 100′ also has more components and hence is more complex as compared to the synchronous dual-track leadframe feeding system 100. As a result, there is a higher risk of machine failure due to malfunction of the devices. More significantly, the high speed movement of the buffer loader 114 and the buffer unloader 116 during alignment generates substantial vibration which significantly limits the speed and accuracy of wire bonding during wire bonding operations since the bonding system 126 and the front and rear leadframe rails 118, 120 are coupled to the two buffer tables. Conversely, the accuracy of the indexing motion is also affected by vibrations of the bonding system 126 during wire bonding.
It is therefore necessary to look into methods to reduce the idling time of the bonding system to increase throughput when bonding leadframes. It is also desirable to devise a system which is sufficiently versatile to carry out wire bonding on leadframes of varying lengths without substantial modification to the system. Eliminating the undesirable effects of vibrations of the various subsystems working together is also important in order to improve the accuracy of wire bonding.